Because the lipidomic profile changes under PGRN-deficient conditions were consistent in both human brain tissues, i.e., disease end-stage, and in mouse embryonic fibroblasts, i.e., physiologically normal, it is highly possible that the study reveals a basic physiologic function for PGRN/GRN.

Given the recent reports on PGRN processing and the presence of mature GRN protein in the lysosomes (Lee et al., 2017; Holler et al., 2017; Zhou et al., 2017), the current study has narrowed the search of GRN protein targets to the metabolic pathways. Mapping out the lysosomal GRN pathway will further improve our understanding of the disease mechanism of PGRN-related disorders.

One is the finding that progranulin deficiency (full-length and granulins) leads to gene-dose alterations in lipid metabolism. The effects on lysosomal lipid processing are very compelling, and offer a brand-new angle on how progranulin affects lipid metabolism. This is an under-studied field that could be of enormous importance in aging and neurodegeneration. The big unanswered question is how PGRN affects lipid metabolism, and what are the downstream effects in different cell types? How is this related to the altered inflammatory phenotype in innate immune cells?

Secondly, the gene dose-dependent effects reported here are unprecedented in mouse cells. In many previous studies, Grn+/– mouse cells have little if any detectable phenotype. The gene dose effect reported here supports the notion that NCL and FTD due to progranulin deficiency are essentially the same disease with shared mechanism (also see Ward et al., 2017). The authors also offered additional evidence with the comparison with the transcriptomes from NPC cells.

From a therapeutic point, this study, along with others, puts targeting lysosomal dysfunction at the center of pharmacologic intervention.

This new paper from Joachim Herz’s lab nicely adds to the mounting evidence that progranulin (PGRN) has an important role in lysosomal function and homeostasis.

Lysosome dysfunction, autophagy impairment, and alterations in lipid metabolism are found in Alzheimer’s disease, frontotemporal dementia, and many other neurodegenerative disorders. Here the authors uncover that complete or partial loss of PGRN in both humans and mice leads to changes in specific lipid profiles.

However, the mechanism whereby decreased or absent PGRN causes lysosome impairment and lipid alterations is unclear. We have recently found that PGRN is rapidly processed into stable granulin proteins (GRNs) in the lysosome, which are haploinsufficient in FTD-GRN patients (Holler et al., 2017). One potential explanation that accounts for both observations is that loss of lysosomal granulins leads to defects in lipid turnover. More exciting work lies ahead to uncover the intricacies of the function of PGRN and GRNs in the lysosome in order to develop therapies where decreased levels of PGRN/GRNs are implicated.

The connection between lysosome dysfunction and Parkinson’s disease is now clear. These papers—as well as one this week by Schapira in Brain and another due out soon from Shulman, Heutink, and the IPDGC—show clearly that lysosome dysfunction, going beyond GBA loss, is key to Parkinson’s pathogenesis. This is indeed an exciting insight.

It is also the case that most pure frontotemporal dementias are caused by different variants in the endosomal/lysosome system. In some sense, one can think of these diseases as formes frustes of lysosome storage diseases.

Less easy to understand is the idea of a “special relationship” between lysosomal storage diseases and dopamine neurons, since we now know there is nothing special about DA neurons in PD. Braaks’ staging and other work shows that the substantia nigra neurons are but one type of neuron hit by the PD process. They are not the first, and the process is by no means selective.